feat: Add governed beneficiary rotation

[Glam26]

closes #249

I have completed the task and pushed the implementation for governed
beneficiary rotation!

Here is a summary of the work done:

1. Repository & Branching: Ensured the work was committed to the
   feature/rotate-beneficiary branch.
2. Feature Implementation ( lib.rs ):
   • Added rotate_beneficiary to the LiquifactEscrow contract with a
   dual-consent requirement ( sme_address.require_auth() and admin.
   require_auth() ).
   • Restricted execution to non-terminal states using guard_status_in
   (via ensure helper for 0 and 1 ).
   • Prevented rotating to the current SME address, introducing the
   EscrowError::NewSmeSameAsCurrent variant.
   • Preserved invariants: after the storage update, only the new SME has
   withdrawal/settlement authority.
   • Published the BeneficiaryRotated event properly tracking old_sme
   and new_sme .
3. Refactoring & Compilation Cleanup:
   • Consolidated EscrowError to remove all the redundant
   #[contracterror] definitions that were causing trait implementation
   conflicts ( E0119 , E0428 ).
   • Initialized tier_lock_secs cleanly across all fund branches to
   resolve a pre-existing E0381 compiler error.
4. Testing ( admin.rs ):
   • Added functional tests: test_rotate_beneficiary_success .
   • Added access/invariant tests:
   test_rotate_beneficiary_same_address_panics and
   test_rotate_beneficiary_wrong_state .
   • Used cargo test test_rotate_beneficiary to isolate and run these
   specific tests successfully, as the rest of the existing test suite has
   pre-existing unclosed delimiter bugs.
5. Documentation: The implementation natively aligns with the
   docs/ESCROW_BENEFICIARY_ROTATION.md specifications that was correctly
   modeled with dual consent.

[drips-wave]

@Glam26 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[mikewheeleer]

well done — in it goes 👍